1. Field of the Invention
The present invention relates to a coordinated communication method and apparatus. More particularly, the present invention relates to a coordinated communication method for cell edge User Equipment (UE).
2. Description of the Related Art
In a cellular communication environment where adjacent cells are operating on the same frequency, cell-edge users may suffer significant interference. Many techniques have been proposed to reduce inter-cell interference and enhance cell edge capacity. The 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) standard supports Inter-Cell Interference Coordination (ICIC). ICIC is a technique that can effectively reduce inter-cell interference by adjusting the transmit power on the frequency resources in the long-term by taking the traffic and interference to the User Equipment (UE) within the cells. The ICIC technique can be classified into two categories: frequency domain ICIC and time-domain ICIC.
In the frequency domain ICIC, plural evolved Node Bs (eNBs) coordinate transmission power patterns among the cells in the frequency domain such that each eNB performs scheduling by taking notice of the coordinated transmission power pattern to mitigate inter-cell interference. In the time domain ICIC, the plural eNBs coordinate the transmit powers and time resources among the cells in the time domain such that each eNB performs scheduling by taking notice of the coordinated time resources to mitigate inter-cell interference. Partial frequency reuse is a technique that can enhance the cell edge performance by taking the frequency reuse rate and inter-cell interference into consideration. In the partial frequency reuse technique, the subcarriers are grouped into sub-bands such that each cell uses some of the sub-bands to mitigate intra-channel interference. In ICIC, the neighbor cells do not transmit signals on a specific frequency resource or reduce transmit power of the signals on the corresponding frequency resource to mitigate the interference to the UEs at the cell edge area. However, there is a limit in improving the throughput by just mitigating inter-cell interference through resource allocation and transmit power adjustment.
As more advanced ICIC techniques, Coordinated Multi-point (CoMP) Transmission/Reception or Multiple Input Multiple Output (MIMO) takes into consideration of the instantaneous channel of the UE at the cell edge (hereinafter referred to as cell edge UE) and traffic condition. Among various CoMP techniques, Coordinated Scheduling/Coordinated Beamforming (CS/CB) CoMP downlink transmission allows each UE to select antenna beamforming of each Base Station (BS or eNB) to improve the cell edge UE throughput when multiple BSs transmit signals to the UE through antenna beamforming. Each UE selects the antenna beamforming of each BS in the course of maximizing the serving BS signal and minimizing the interference from neighbor eNBs. The CS/CB CoMP BSs transmit data to the UE within their own cells but not to the UE within the neighbor coordinated cell.
In Joint Processing/Transmission (JPT) CoMP, the adjacent eNBs transmit the same information to a cell edge UE simultaneously to enhance the cell edge UE throughput. In order to increase the entire cell throughput, plural eNBs are capable of transmitting the user signals to plural UEs simultaneously. Accordingly, the JP CoMP eNBs transmit data to the UEs within the neighbor CoMP cells as well as the UE within their own cells. CoMP is the technique that is capable of minimizing inter-UE or inter-eNB interferences which maximizes throughput in such a way that logically all eNBs share the channel information between multiple transmit and receive antennas of the multiple eNBs and multiple UEs. The CoMP technique is superior to the ICIC technique in cell edge and cell average throughput. However, the CoMP technique has problems of large information amount exchanged over backhaul and CoMP signal processing complexity.
However, due to achievability issues caused by the distortion of the channel information according to the backhaul overhead and backhaul delay for the channel information exchange, only the Multi User MIMO has been adopted to the LTE release 10. In LTE release 11, the CoMP techniques are under discussion as a study item in view of performance gain and standardization range. There are four CoMP scenarios considered in LTE-Advanced (LTE-A) release 11 according to the LTE communication standard TS 36.912.
FIG. 1 is a diagram illustrating four CoMP scenarios according to the related art.
Referring to FIG. 1, the first scenario 110 is of Intra-eNB CoMP in homogeneous deployment, and the second scenario 130 is of Inter-eNB CoMP in homogeneous deployment. The third scenario 150 is an Inter-cell CoMP in heterogeneous deployment, and the fourth scenario 170 is a Distributed antenna system with shared cell ID.
In the fourth scenario 170, the eNBs share the control channel Physical Downlink Control Channel (PDCCH)/Physical Uplink Control Channel (PUCCH) including CRS or resource allocation information and use the data channel Physical Uplink Shared Channel (PUSCH)/Physical Downlink Shared Channel (PDSCH) in a spatially distributed manner. In this way, it is possible to reduce control channel error rate at the cell edge area and improve the data channel throughput with MIMO and/or Space-Division Multiple Access (SDMA).
The LTE release 8/9 supports the Inter-Cell Interference Control/Coordination (ICIC) as a technique to guarantee the cell edge UE performance.
The downlink ICIC is a proactive technique to guarantee the cell edge UE throughput using Relative Narrowband Transmit Power (RNTP) information with inter-band power information for cell edge UE and inner UE.
FIG. 2 is a diagram illustrating an architecture of radio access systems connected through X2 interfaces according to the related art.
Referring to FIG. 2, the radio access systems are the eNBs 221, 222, and 223. Each eNB forms a cell. For example, the eNB 223 forms the cell 213. The eNBs communicate with each other through X2 interfaces 231, 232, and 233.
In order to perform uplink ICIC, the eNBs 221, 222, and 223 exchange information through X2 interfaces 231, 232, and 233. The information exchanged among the eNBs may include at least one of High Interference Indicator (HII) for the proactive mode and Overload Indicator (OI) for the reactive mode.
HHI is information used in the proactive mode for the eNB 221 to notify the neighbor cells of the high interference to be occurred in advance. Typically, in the power control policies under discussion in the LTE standard, e.g. Fractional Power Control and Transmit Power Control (TPC) Command in PDCCH, the eNB knows the power of the UE. Accordingly, the eNB is able to set the indicator to 0 or 1 based on a certain threshold to UE's transmit power for each Physical Resource Block (PRB). The indicator is set to 0 to indicate low interference, and is set to 1 to indicate high interference. At each ICIC period, the neighbor cells receiving the HII allocate resources to the UE in the course of avoiding collision with the Resource Block (RB) indicated by the HII information. The HII information may be transmitted along with a Target Cell ID. In this case, the target cell for the bit indicating the PRB interference strength level is designated. If the HII with no designation of a specific target cell, this may degrade the scheduling gain.
FIG. 3 is a diagram illustrating an architecture of radio access systems using the OI information according to the related art.
Referring to FIG. 3, the mobile communication system includes a first eNB 310, a second 320, and a third eNB 330. A second cell edge UE 322 is located at the coverage boundaries of the first and second eNBs 310 and 320. A second inner UE 321 is located in the inner zone (far from the boundary) of the second eNB 320. A third cell edge UE 332 is located at the coverage boundary of the first and third eNBs 310 and 330. A third inner UE 331 is located in the inner zone of the third eNB 330.
The first eNB 310 measures the interference strength on each RB and notifies the neighbor cells 320 and 330 of the measurement result using Overload Indicator (OI) through X2 interface. OI is the reactive mode information. The most interference to the first eNB 310 is caused by the cell edge UEs 322 and 332. According to the current LTE standard, the first eNB 310 sets the RB-specific interference level to one of High, Medium, and Low, and notifies the neighbor cells (or eNBs) 320 and 330 of the interference level. An uplink modem measures the thermal noise level No per PRB across the give bandwidth at every ICIC period. The first eNB 310 retains the OI information transmitted most recently to the neighbor cells (or eNBs) 320 and 330 and, if the OI value of the RB has changed, broadcasts the OI information to the neighbor cells (or eNBs) 320 and 330. The Interference over Thermal noise (IoT) control technique for adjusting the powers of the cell edge UEs on the same bandwidth based on the neighbor cell interference measurement can be used to guarantee the cell-specific uplink cell coverage. Like the HII, the OI information of the reactive mode can be used in uplink scheduling.
Although the detailed implementation may be modified somewhat, ICIC is capable of enhancing the cell edge UE performance by basically reflecting the ICIC standard information (HII and OI) based on the interference measurement to the uplink scheduling. However, the HII or IOI information is limited significantly in bit size and does not reflect the uplink channel gain and interference characteristics enough. Accordingly, the ICIC function is restrictive from the viewpoints of both the UE throughput enhancement and entire cell throughput enhancement.
FIG. 4 is a diagram illustrating graphs of long term pilot strength information in downlink and uplink of a system operating in the fourth scenario of FIG. 1 according to the related art.
Referring to FIG. 4, the macro cell A and the RRH A share a cell identifier. Likewise, the macro cell B and the RRH B share a cell identifier.
According to the fourth scenario 170 of Distributed antenna system with shared cell ID, the macro cell and the Remote Radio Head (RRH) use the same cell ID. Accordingly, the Reference Signal Received Power (RSRP) signals of the RRHs overlap at the RRH cell boundary areas. As a consequence, each cell cannot acquire the information, for use in the inter-cell coordinated scheduling by taking notice of uplink interferences among the UEs at RRH cell edge area, from the RSRP signal. This means that the HII-based ICIC scheduling according to the related art can be used for uplink inter-cell coordinated scheduling only for the cell edge UEs located in the macro cell handover area or macro cell edge area due to the uplink scheduling. Accordingly, the UEs located at the boundary between the sub-cells of the RRHs connected to the macro cell A through fiber optic lines suffer significant uplink interference from the viewpoint of uplink.
Without using uplink channel information (SRS), it is impossible to control the inter-UE interference at RRH sub-cell boundary areas or guarantee IoT. Accordingly, the uplink coverage enhancement is restricted. If the number of eNBs per unit area increases through the cell planning without overcoming this drawback, this causes loss in view of Capital Expenditure (CAPEX) and Operation Expense (OPEX).
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.